(1) Flatworms are unsegmented, bilaterally symmetrical worms that lack a coelom but that do have three germ layers.(2) The coelom extends into the rays, where the gonads are located.(3) The ophiuroid coelom is strongly reduced, particularly in comparison to other echinoderms.(4) In mellitids, the intestine is separated from the rest of the coelom by a well-defined wall of the peripheral ballast system.(5) Except in leeches, the coelom is partially subdivided by septa.(6) The genes have an arrangement relatable to those of coelomate metazoans, but not to those of secernentean nematodes.(7) The reproductive system is composed of a single saccular gonad in a coelomic pouch lying dorsal to the digestive system.(8) The soft parts in the axial canal of the stem consist of nerve cords, coelomic cavities, and associated organs which require more metabolic maintenance.(9) Schizocoelous mesoderm formation in coelomate spiralians also makes use of individual cell immigration during its early phase.(10) They first evolved in the earlier Cambrian period, as part of the great radiation of coelomate animals that occurred at that time.(11) A number of other volatile, low molecular mass compounds were found in the coelomic fluid of these and other nereid species and thought to play a role in their reproduction.(12) Such a radical change in the mitochondrial gene arrangement within a closely related taxon has never been observed in other coelomate phyla.(13) These characters would have facilitated volume changes of the coelomic cavity due to dorso-ventral rotation of the u2018synsacrumu2019 on the dorsal vertebrae.(14) Embryonic coelomic structures have specific fates as the bilaterally symmetrical larvae metamorphose into radially symmetric adults.(15) This animal would have to be relatively large since the coelomate condition cannot have evolved in a tiny animal.
